This invention relates to gas separation polymeric membranes of amorphous aryl substituted polyarylene oxide and to apparatus and processes utilizing such membranes for selectively separating at least one gas from a gaseous mixture by permeation. More particularly, it relates to membranes of amorphous aryl substituted polyphenylene oxide which is capable of being formed into hollow fiber membranes by extruding a polymer solution into an aqueous coagulation bath.
The viability of the use of membranes for fluid separations as compared to other separation procedures such as absorption, adsorption, and liquefaction often depends on the cost of the apparatus and its operation including energy consumption, degree of selectivity of separation which is desired, the total pressure losses caused by the apparatus for conducting the separation procedure which can be tolerated, the useful life of such apparatus, and the size and ease of use of such apparatus. Thus, membranes are sought which provide desired selectivities of separation, fluxes and strength. Moreover, in order to be commercially attractive on an economic basis, the membranes are preferably capable of being manufactured in large quantities while achieving a reliable product quality and being readily and relatively inexpensively assembled in a permeator. Particularly advantageous membranes are anisotropic hollow fiber membranes which have a relatively thin layer (often referred to as separating layer, barrier layer, or active layer) integral with a porous structure which provides support to the separating layer and offers little, if any, resistance to the passage of fluids. In order to prepare these integral anisotropic membranes, a unitary membrane structure must be formed which possesses diametrically opposed structures. The separating layer must be formed such that it is thin and possesses few, if any, pores or other defects. On the other hand, the conditions which make the integral anisotropic membrane must also provide a support structure which is highly open such that it offers little resistance to fluid flow.
Membranes have been prepared in film and in hollow fiber form. Numerous proposals have been made pertaining to the preparation of integral anisotropic membranes in film form. In general, anisotropic film membranes are prepared by casting a solution of the polymer to form the membrane in a solvent onto a surface, e.g., a polished glass surface. The polymer may be allowed to coagulate, at least partially, in air or a gaseous or vaporous environment and then it is usually immersed into a liquid coagulant. Considerable flexibility exists in preparing anisotropic film membranes. For instance, since the polymer solution is placed on a support, the membrane precursor structure need not be self supporting at least until after coagulation is completed. Similarly, since one surface of the cast membrane is in contact with the support, each side of the membrane may be subjected to different coagulation conditions thereby permitting substantially different structures to be achieved at each surface of the membrane. Accordingly, membranes having a relatively thin layer having an essential absence of pores may be achieved at one surface of the film membrane, while the remainder of the membrane may be relatively porous. Moreover, since the film membrane precursor is supported, the coagulation conditions including coagulation times, can be widely varied to achieve the desired film membrane structure.
In some instances, however, film membranes may not be as attractive as other gas separation apparatus due to the need for film membranes to be supported to withstand operating conditions and the overall complexity of apparatus containing film membranes. Membranes in the configuration of hollow fibers may overcome some of the deficiencies of film membranes for many separation operations. The hollow fibers are generally self-supporting even under operating conditions, and can provide a greater amount of membrane surface area per unit volume of separation apparatus than that which may be provided by film membranes. Thus, separation apparatus containing hollow fibers may be attractive from the standpoint of convenience, in size and reduced complexity of design.
Many different considerations are involved in making a hollow fiber membrane than are involved in making a film membrane. For instance, no solid support, or interface, can be provided in a process for spinning a hollow fiber membrane. Moreover, in spinning procedures, the polymer solution must be of sufficient viscosity to provide a self-supporting extrudate prior to and during coagulation, and the coagulation must be quickly effected after extrusion such that the hollow fiber membrane is not adversely affected.
Processes for the formation of integral anisotropic membranes must not only meet the criteria for forming integral anisotropic hollow fiber membranes but also must be compatible with hollow fiber spinning capabilities. Hence, many constraints are placed upon the techniques available to produce integral anisotropic hollow fiber membranes. Commonly, in hollow fiber membrane spinning procedures, a solution of the polymer to form the hollow fiber membrane in a solvent is extruded through a spinnerette suitable for forming a hollow fiber structure, and a gas or liquid is maintained within the bore of the hollow fiber extrudate such that the hollow fiber configuration can be maintained. The hollow fiber extrudate must quickly be coagulated, e.g., by contact with the non-solvent for the polymer, such that the hollow fiber configuration can be maintained. The hollow fiber spinning process contains many variables which may affect the structure, or morphology, of the hollow fiber membrane such as the conditions of the polymer solution when extruded from the spinnerette, the nature of the fluid maintained in the bore of the hollow fiber membrane extrudate, the environment to which the exterior of the hollow fiber extrudate is subjected, the rapidity of coagulation of the polymer in the hollow fiber extrudate, and the like.
In order for a procedure to be attractive for the production of commercial quantities of membranes, it is also desired that the procedure be safe and economical. Thus, the solvent should not be unduly toxic, and advantageously, the solvent exhibits a very low vapor pressure to minimize risk of inhalation and/or air pollution. Moreover, a solvent having a very low vapor pressure may also minimize the risk of explosion and fire. Furthermore, waste materials from the spinning process should be able to be economically and safely discarded or recycled.
Since the solvent is only one component used in the spinning procedure, other components such as fluid within the bore of the hollow fiber extrudate, non-solvent to assist in effecting coagulation, washing fluids to remove solvent from hollow fiber membranes, and the like should also be economical and safe. Heretofore proposals have been made to use, e.g., gasoline, kerosene or other hydrocarbonaceous materials in the spinning procedure either as coagulants or to assist in drying such as disclosed by Arasaka et.al., in U.S. Pat. No. 4,127,625. Such materials clearly pose toxicity and fire risks as well as disposal problems. Moreover, in the quantities required to effect, e.g., coagulation, washing, etc., the expense of the hydrocarbonaceous materials could be a factor in the economics of the spinning process. Accordingly, it is desired to use highly safe, readily available materials, such as water or aqueous solutions, wherever possible in the spinning process, especially as non-solvent to assist in effecting coagulation and in washing to remove solvent from the hollow fiber membrane. The ability to use water, of course, will depend to a large extent upon the properties of the polymer solution with respect to water, i.e., solubility in water, heat of dilution in water, stability in water, and the like.
Polyarylene oxides have been recognized as material of some potential in the membrane separation field. For instance, Robb in U.S. Pat. No. 3,350,844 disclosed that polyarylene oxide membranes, for instance membranes of 2,6-dimethylphenylene oxide membranes, have unique properties such as a high separation factor and flux together with strength and ability to form thin films. Robb further discloses that factors such as temperature, pressure, elongation of oriented membrane material, the amount of crystallinity, among others, in the polyarylene oxide resin, may effect permeability.
In this regard polyphenylene oxide resins have a low resistance to most common organic solvents. Aromatic and chlorinated hydrocarbon solvents dissolve polyphenylene oxide polymers, while other solvents and solvent vapors induce crazing in molded polyphenylene oxide parts under stress thus causing almost complete loss of strength.
See also Kimura, U.S. Pat. Nos. 3,709,774; 3,762,136; and 3,852,388 which relate to membranes of polyxylene oxide with the same apparent disadvantages. In this regard Kimura discloses dry asymmetric membranes comprising a porous layer of interconnected crystals of a polyarylene oxide. The membranes are in the form of films cast from a polymer solution.
An alternative form of polyarylene oxide membranes is disclosed by Salemme in U.S. Pat. No. 3,735,559 where various ionic forms of a sulfonated polyxylene oxide membrane are disclosed. Among the disadvantages discussed are that it is necessary to preshrink such membranes to avoid rupturing; the hydrogen ion form is unstable and may undergo sulfone formation resulting in crosslinking or may, in the presence of water, undergo hydrolysis with the liberation of sulfuric acid; various counter ion salt forms of the membrane are stable and will avoid detrimental crosslinking but such membranes may densify in the presence of water.
Henis et.al. in U.S. Pat. No. 4,230,463 disclosed multicomponent membranes for gas separations which comprised a coating in contact with a porous separation membrane where the separation properties of the multicomponent membranes are principally determined by the porous separation membrane as opposed to the material of the coating. Henis et.al. in Examples 59-61 disclosed such multicomponent membranes where the porous separation membrane comprised brominated poly(xylene oxide) polymer where the bromination was essentially upon methyl groups. The membranes were in hollow fiber form. Such brominated poly(xylylene oxide) polymer is disadvantageous in that the polymer exhibits intrinsic permeability significantly lower than the intrinsic permeability of the precursor polymer, poly(xylylene oxide) also known as poly(2,6-dimethyl-1,4-phenylene oxide).
In summary suitable amorphous polyarylene oxide membranes have not been provided in hollow fiber form for gas separations which can exhibit sufficient flux and selectivity of separation for general commercial operations in the presence of adverse environmental conditions as the presence of chemical contaminants, extremes of differential pressure and temperature.